Heat treatment of particulate materials

ABSTRACT

A particulate material is heat treated, e.g. calcined, by being passed through a fluidised bed comprising particles of an inert refractory material. Throughout the period in which the particulate material is passing through the fluidised bed, the fluidised bed contains a major proportion by weight of the inert refractory material. The size, shape and density of the particles of inert refractory material and the velocity of the fluidising gas are such that the particles of inert refractory material are retained in the fluidised bed while the particulate material being heat treated is carried through the fluidised bed. The particulate material is retained within the fluidised bed for a time sufficient to effect the desired heat treatment thereof.

United Statesv Patent 1 1 3,775,040 Vollans Nov. 27, 1973 HEAT TREATMENTOF PARTICULATE 3,022,989 2/1962 Pyzel 263/21 A x MATERIALS 3,349,50010/1967 Woll 34/10 Inventor: Edgar Colin Vollans, St. Austell,

England English Clays Lovering Pochin & Company Limited, Cornwall,England Filed: June 29, 1971 Appl. No.: 157,991

Assignee:

Foreign Application Priority Data July 1, 1970 Great Britain 32,025/70 7References Cited UNITED STATES PATENTS Primary Examiner-William F. O'DeaAssistant Examiner-Harold Joyce Attorney-Stevens, Davis et al.

[5 7 ABSTRACT A particulate material is heat treated, e.g. calcined, bybeing passed through a fluidised bed comprising particles of an inertrefractory material. Throughout the period in which the particulatematerial is passing through the fluidised bed, the fluidised bedcontains a major proportion by weight of the inert refractory material.The size, shape and density of the particles of inert refractorymaterial and the velocity of the fluidising gas are such that theparticles of inert refractory material are retained in the fluidised bedwhile the particulate material being heat treated is carried through thefluidised bed. The particulate material is retained within the fluidisedbed for a time sufficient to effect the desired heat treatment thereof.

10 Claims, 4 Drawing Figures EAHNIHINUVPY mm 7 3775040 SHEET 30F 4 HEATTREATMENT OF PARTICULATE MATERIALS This invention relates to the heattreatment of particulate materials and, more particularly butnotexclusively, is concerned with the calcining of minerals such as thesilicates of aluminium and of the alkaline earth metals.

The heat treatment of minerals such as the silicates of aluminium and ofthe alkaline earth metals by an operation known as shock calcination isdisclosed in British Patent specifications Nos. 866,326 and 869,966 andessentially comprises the spraying of the silicates into a vortex of hotgases. These known processes, however, suffer from the disadvantage thatthe temperature varies within the reaction vessels and cannot beaccurately controlled. Furthermore, the residence time of particles inthe reaction vessels varies over a wide range and there is a strongtendency for the particles to agglomerate so that the particle size ofthe product also varies over a wide range.

It is an object of the present invention to provide a process for theheat treatment of particulate materials in which the above disadvantagescan be overcome or inexacerbated.

According to the present invention there is provided a process for heattreating, e.g. calcining, a particulate material, which processcomprises passing the particulate material through a heated fluidisedbed comprising particles of an inert refractory material, wherein (i)the fluidised bed contains throughout the period in which theparticulate material is passing therethrough a major proportion byweight of said inert refractory material; (ii) the size, shape anddensity of the particles of inert refractory material and the velocityof the fluidising gas are such that the particles of inert refractorymaterial are retained in the fluidised bed while the particulatematerial is carried through the fluidised bed; and (iii) ,theparticulate material is retained within the fluidised bed for a timesufficient to effectthe desired heat treatment thereof.

Preferably, the inert refractory material consists of particles having adefinite but narrow particle size distribution range, it beingadvantageous for the range of particle size of the inert refractorymaterial tobe such that the coarsest particles of the inert refractorymaterial are not more than four times larger than the finest particlesof the inert refractory material. It is also preferable for the shapeand average particle size of the inert refractory material to be suchthat the .inear gas velocity necessary to fluidise the inert refractorymaterial is at least five times that necessary to convey the particulatematerial out of the fluidised bed.

The inert refractory material can be, for'example, sand, silica or aceramic material. Good results have been obtained when using as theinert refractFy material a calcined kaolin clay, for example that s ldunder the Trade Mark MOLOCHITE. When th particulate material to be heattreated consists pred minantly of particles smaller than 50 micronsequivale t spherical diameter, the inert refractory material ispreferably selected from particles having a diameter in the range offrom 0.5 to 5.0 mm.

Advantageously, the particles of the particiilate material to be heattreated are fed pneumatically into the fluidised bed; but other feedmethods, for example gravity feed, can alternatively be employed. Ii hasalso been found to be advantageous to feed the particles of theparticulate material to be heat treated via a conduit passinglongitudinally through the top of the fluidised bed reactor andextending into the fluidised bed, although laterally positioned conduitshave also functioned satisfactorily. The fluidised bed is advantageouslyof the dense-phase type, but spouting fluidised beds can also be used.

A fluidised bed is formed when a fluid flows upwardly through a bed ofsuitably sized solid particles at a velocity sufl'iciently high to buoythe particles, to overcome the influence of gravity thereon, and toimpart thereto an appearance of great turbulence.

The fluidising gas used to form the fluidised bed can be air, andadvantageously the fluidising gas is also used to support the combustionof a fuel which provides the heat for the fluidised bed. The heating ofthe fluidised bed can be achieved, for example, by using hot combustiongases as the fluidising gas or by burning a fuel within the fluidisedbed itself. This latter method is preferred because it allows the use ofa fluidisation grid, or distribution plate, which does not need towithstand very high temperatures, as well as giving a more eventemperature distribution within the fluidised bed. This allows a moreevenly heat treated, e.g. calcined, product to be obtained. Morepreferably, the fluidised bed is initially heated by passingtherethrough the combustion gases obtained from a conventionalcombustion chamber burning a liquid, gaseous or powdered solid fuel,and, when the temperature of the fluidised bed approaches the desiredworking temperature, the supply of combustion gases is stopped and fuelis injected into the fluidised bed, or at a point just below thefluidisation grid, so that ignition and combustion of the fuel takesplace in the fluidised bed itself. The fuel injected into the fluidisedbed can be liquid, gaseous or a powdered solid. One or more furtherfluidised beds can be provided above the first fluidised bed to ensurecomplete combustion of all the fuel injected into the lower bed.

Generally, the particles of the particulate material which are fed intothe fluidised bed reactor should be dry or substantially dry. However,in certain circumstances, for example when the particulate material thatis to be heat treated is to be used to form a cement, a slurry of theparticulate material can be fed into the fluidised bed. This latter modeof operation, however, requires a high degree of heating if hightemperatures are to be maintained within the fluidised bed.

Generally, if the particulate material to be heat treated is to becalcined, the fluidised bed will be operated at a temperature in therange of from 600 to l,200 C. When calcining silicates of the alkalineearth metals or of aluminium, for example, the average residence time ofthe silicate particles within the heated fluidised bed should generallybe 1 second or less. The residence times of individual particles of aparticulate material within the fluidised bed should not normally exceed3 seconds. The residence time of the particulate material in thefluidised bed is governed by the bed depth and the velocity of the gasespassing up through the bed; the smaller the bed depth and the greaterthe gas velocity, the shorter will be the residence time of theparticulate material in the fluidised bed. The gas velocity is affectedin turn by the size, specific gravity and shape of the particles ofinert refractory material, since there is a minimum gas velocityrequired to fluidise the particles. For particles of a given specificgravity and shape, the fluidising velocity is less for small particlesthan for large particles.

Separation of the heat treated particulate material from the fluidisinggas can be performed by known methods, for example by using a cyclone ortwo or more cyclones arranged in series or in parallel. Other types ofseparator, for example electrostatic precipitators or bag filters, mayalso be used either alone or in conjunction with a cyclone or cyclones.The suspension of heat treated particulate materials in the fluidisinggas is preferably cooled to below 300 C. either before or after passingthrough the separator(s) by injecting a cold fluid, e.g. water or air,into the suspension. It is advantageous to provide an exhaust fan whichwill draw I the suspension through the separator and maintain thepressure in the reaction vessel very slightly below atmospheric pressureso that the particulate material and inert refractory material are notblown out through the feed conduit.

The process of the present invention enables close control of thetemperature at which the particulate material is treated to be obtainedand allows efficient heat transfer between the particulate material andthe inert refractory material. In addition, the turbulence within thefluidised bed tends to minimise the tendency of the particulate materialbeing heat treated to agglomerate.

For a better understanding of the invention, and to show how the samecan be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings, which show four embodiments of anapparatus for use in the process of the present invention. Similarreference numerals in the Figures refer to similar parts of theapparatus.

Referring first to FIG. 1, a fluidised bed of particles of inertrefractory material 1 is supported on a perforated distribution plate 2and is contained in a vessel 3 having thermally insulated walls 4. Thefluidised bed is initially heated to near the working temperature bymeans of hot combustion gases from a combustion chamber 5 in which afuel oil is burned in air. When the desired fluidised bed temperaturehas been reached, the supply of hot combustion gases is stopped and fueloil is injected through a conduit 6 below the distribution plate of thefluidised bed, or through a conduit 7 above the distribution plate, orboth. The particulate material which is to be heat treated, for examplean uncalcined silicate, is injected as a suspension in air into thefluidised bed thorugh a conduit 8. Heat treated, e.g. calcined,particulate material leaves the apparatus through a conduit 9 whichcommunicates with a cyclone separator 10. The gases leave the cyclonethrough a conduit 11 and the heat treated particulate material collectsin the bottom of the cyclone whence it can be removed by opening, inturn, valves 12 and 13. A sight glass 14 is provided in the top of thevessel 3 to permit visual inspection of the fluidised bed, and adischarge conduit 15 permits discharge of the particles of inertrefractory material from the vessel.

Referring next to FIG. 2, there is shown an embodiment of the inventionin which the particulate material fluidised bed comprises a section ofrelatively small cross-sectional area 31, a section of relatively largecross-sectional area 32 and a transitional section 33 of frusto-conica]shape. This arrangement causes a decrease in the linear velocity ofgases above the upper surface of the fluidised bed 1 when the apparatusis in use so that any oversize particles of particulate material beingheat treated, or any particles of inert refractory material which havebeen carried out of the fluidised bed, are returned to the fluidisedbed.

Referring next to FIG. 4, there is shown an embodiment of the inventionin which a vessel 4 contains two fluidised beds la and lb which aresupported on perforated distribution plates 2a and 2b respectively. Fuelfor combustion in the vessel, e.g. fuel oil, can be injected into eitheror both of the fluidised beds through conduits 7a and 7b. Theparticulate material which is to be heat treated is injected, preferablyin suspension in air, into either or both of the fluidised beds throughconduits 8a and 8b. Discharge conduits 15a and 15b permit discharge ofthe inert refractory material from the vessel.

The invention is further illustrated by the following Examples.

EXAMPLE 1 A kaolinitic clay mineral, from which particles having anequivalent spherical diameter greater than 5 microns had been removed bygravitational and centrifugal separation and which comprised percent byweight of particles smaller than 2 microns equivalent sphericaldiameter, was fed pneumatically into the apparatus described above withreference to FIG. 1 of the accompanying drawings. The apparatuscontained a fluidised bed having a diameter of 10% inches and containingapproximately 30lbs of MOLOCHITE. The MOLOCI-IITE had a particle sizedistribution such that substantially all the particles passed thorugh a/ath of an inch mesh sieve and were retained on a No. 8 mesh BritishStandard sieve. The pressure drop across the fluidised bed was 10 incheswater gauge. The initial inlet temperature of combustion gases to thefluidised bed was l,480 C., and the operational temperature of thefluidised bed was l,l00 C. Gas flow through the fluidised bed was 78cubic feet per minute, and the feed rate of ground kaolinitic claymineral was 61 lbs/hr. Under these conditions, the calcined material hadthe properties noted in Table I below, which are compared with theinitial properties of the kaolinitic clay mineral.

TABLE I Property Before After Treatment Treatment Valley abrasion value30 lOl Specific gravity 2.64 2.l4 Loss on ignition (weight 13.1 0.2Percent by weight smaller than 2 microns 80 22 Brightness 89.5/93.089.l/9l.9

" Measured as reflectance to light of 458 "Ill/574 mp. wavelength on anElrepho brightness meter.

EXAMPLE 2 A kaolinitic clay mineral similar to that used in Example 1above was treated as described in Example 1, except that the initialinlet temperature of combustion gases to the fluidised bed was l,l80 C.,the bed temperature was 900 C., the air flow through the fluidised bedwas 90 cubic feet per minute, and the feed rate of kaolinitic claymineral was 55 lbs/hr. The results obtained are set out in Table 11below.

TABLE II Property Before After Treatment Treatment Valley abrasion value30 101 Specific gravity 2.64 2.30 Loss onignition (weight 13.1 0.5Percent by weight smaller than 2 microns 80 19 Brightness 89.5/93.086.7/90.7

* Measured as reflectance to light of 458 mp./574 my. wavelength on anElrepho brightness meter.

EXAMPLE 3 A kaolinitic clay mineral similar to that employed in Examples1 and 2 above was treated as described in Example l, except that theinlet temperature of combustion gases to the fluidised bed was 920 C.,the bed operating temperature was 700 C., the air flow through thefluidised bed was 108 cubic feet per minute, and the feed rate ofkaolinitic clay mineral was 65 lbs/hr. The results obtained are set outin Table III below.

* Measured as reflectance to light of 458 nip/574 mp wavelength on anElrepho brightness meter.

EXAMPLE 4 A kaolin clay comprising 80 percent by weight of particlessmaller than 2 microns equivalent spherical diameter was treated asdescribed in Example 1 above, except that the initial inlet temperatureof combustion gases to the fluidised bed was 1,250 C., the bedtemperature was l,000 C., the air flow through the fluidised bed was 85cubic feet per minute, and the feed rate of kaolin was 55 lb/hour. Thecalcined product was air classified and the fine product of theclassification process was found to have a particle size distributionsuch that 34 percent by weight consisted of particles smaller than 2microns equivalent spherical diametr, 8 percent by weight of particleslarger than 10 microns equivalent spherical diameter and 0.02 percent byweight of particles larger than 53 microns. The specific gravity was2.15, the loss on ignition 0.647 percent by weight, the percentagereflectance to light of 458 nm, wavelength was 86.5 and to light of 574nm. wavelength was 89.8. This material is referred to hereinafter asFiller A.

A second mineral filler, hereinafter referred to as Filler B," was akaolin clay which had been calcined in a conventional multiple hearthfurnace at a maximum temperature of 950 C. for a total time of 4 hoursand the final product has a particle size distribution such that 10percent by weight consisted of particles having an equivalent sphericaldiameter larger than 10 microns and 50 percent by weight consisted ofparticles having an equivalent spherical diameter smaller than 2microns. The specific gravity was 2.50 and the percentage reflectance tolight of 458 nm wavelength was 88.0.

Each of the two fillers A and B were incorporated into butyl rubbercompounds A and B according to the recipes given in Table IV below:

Table IV Parts by weight Component compound compound Butyl rubber (0.65mole unsaturation) 1 Process oil 5 Paraffin Wax 5 Zinc oxide 5 Sulphurp-quinone dioxime dibenzoate Red lead Filler A Filler B Batches of eachof the two compounds were cured in a steam heated press at C. for 40minutes and the cured sheets were subjected to tests for modulus at 300percent elongation, tensile strength, percent elongation at break,hardness and tear strength. The tests were all carried out in accordancewith E5. 903 and the results are given in Table V below.

TABLE V Property compound compound Modulus at 300% elongation Tensilestrength Elongation at break 555 515 Hardness lRHD 60 58 Tear strengthlbf (kgf) 1 1.1

These results show that there is no significant difference in mechanicalproperties between Compound A and Compound B. However, Filler A has alower specific gravity than Filler B and therefore a smaller weight isrequired to fill a given volume. Rubber fillers are generally bought byweight and therefore a saving in cost may be effected. In additionFiller A has a slightly different refractive index from that of therubber matrix and therefore has some pigmenting properties. Filler B hasa refractive index which is substantially the same as that of the rubbermatrix.

The cured compounds were also subjected to tests for electricalinsulation properties after immersion in water at 50 C. in accordancewith BS. 2899 part 3. The results are shown in Table VI below.

TABLE VI compound compound A B increase in capacitance after immersionat 50C for l- 14 days 7.1 5.3 7 14 days 5.3 2.5 Permittivity after 14days immersion at 50C. 4.3 4.1 Power factor after 14 days immersion at50C. 0.01 1 0.011

There is no significant difference between the results for Compound Aand Compound B.

EXAMPLE A kaolin clay comprising 80 percent by weight of particlessmaller than 2 microns equivalent spherical diameter was treated asdescribed in Example 1 above, except that the initial inlet temperatureof combustion gases to the fluidised bed was l,250 C., the bedtemperature was l,000 C., the air flow through the fluidised bed was 85cubic feet per minute, and the feed rate of kaolin was 55 lb/hr.

The final product had a specific gravity of 2.2, a loss on ignition of0.025 percent by weight, a percentage reflectance to light of 458 nm.wavelength of 87.2 and to light of 574 nm. wavelength of 91.9 and aparticle size distribution such that 23 percent by weight consisted ofparticles smaller than 2 microns equivalent spherical diameter, 12percent by weight consisted of particles larger than microns equivalentspherical diameter and 0.03 percent by weight consisted of particleslarger than 53 microns. This material is hereinafter referred to asExtender A.

A second mineral material, hereinafter referred to as "Extender B, was akaolin clay which had been flash calcined in a conventional manner bypassing the clay in powder form rapidly through an oil-fired combustionchamber at 850 C. The material had a specific gravity of 2.0 and aparticle size distribution such that 35 percent by weight consisted ofparticles smaller than 2 microns equivalent spherical diameter.

Two exterior grade emulsion paints were made up according to thefollowing recipe. Paint A contained Extender A and Paint B containedExtender B.

by weight Rutile titanium dioxide 20.6 Extender A or B l2.6 2% solutionof hydroxy ethyl cellulose l7.4 5% dispersant solution 3.9 Butylcarbitol acetate l.0 Water 109 Vinyl acetate/vinyl versatic acid estercopolymer at 50% by weight solids 33.5

Small quantities of anti-freeze, rust inhibitor, fungicide and defoamer.

The pigment volume concentration was 40 percent. The two paints weresubjected to tests for film brightness, dispersion and opacity (contrastratio) and the results are given in Table Vll below.

TABLE VII Property Paint Paint B Film brightness Dispersion Opacity(contrast ratio) 93.0 92.3

Notes:

1. The film brightness test comprised applying a film of the paint at aconstant thickness of 0.0032 inch (0.081 mm) to a sheet of MELINEXplastics material and cutting five discs from the coated sheet. The fivediscs were stacked and five measurements of the brightness of the topdisc of the stack were made using an ELREPHO brightness meter with lightof 457 nm. wavelength. Between each measurement the bottom disc of thestack was transferred to the top, and the average of the fivemeasurements was calculated.

2. The dispersion test was performed by drawing down each paint on aHegman gauge. Both paints had previously been mixed in the samelaboratory SILVER- SON shrouded-impeller mixer for a standard time of 25minutes.

3. The opacity test comprised applying a film of the paint at a constantthickness of 0.0032 inches (0.08] mm) using a draw bar over Morestcharts No.102 which have equal black and white portions. The percentagereflectance (MgO percent) to light of 540 nm wavelengths of the filmover the black and white portions were measured with an ELREPHObrightness meter. The contrast ratio was calculated by the formula:contrast ratio reflectance over black portion/% reflectance over whiteportion X100 Additional qualitative tests were performed to compare thebrushability, flow, visual opacity, visual brightness and can stabilityof the two paints. Paint A was observed to have superior brushability,flow, visual opacity and visual brightness properties to Paint B, butthe two paints had similar good can stability.

I claim:

1. A process for heat treating a particulate material which is asilicate of aluminum or of an alkaline earth metal and which essentiallyconsists of particles smaller than 50 microns equivalent sphericaldiameter, which process comprises passing the particulate materialthrough a fluidized bed heated to a temperature in the range 600 C. tol,200 C., wherein the fluidized bed comprises particles of an inertrefractory material having diameters in the range of from 0.5 to 5.0millimeters a particle size distribution such that the coarsestparticles of the inert refractory material are not more than four timeslarger than the finest particles of the inert refractory material,wherein throughout the period in which the particulate material ispassing through the heated fluidized bed the inert refractory materialconstitutes a major proportion by weight of said fluidized bed, andwherein the size, shape and density of the particles of the inertrefractory material are such that the linear gas velocity necessary tofluidize the inert refractory material is at least five times thatnecessary to convey out of the fluidized bed the particulate materialwhich is to be heat treated whereby the particles of inert refractorymaterial are retained in the fluidized bed while the particulatematerial to be treated is carried through the fluidized bed and iswithin the fluidized bed for an average residence time of not more thanone second.

2. A process according to claim 1 wherein the inert refractory materialis selected from the group consisting of sand, silica and ceramicmaterials.

3. A process according to claim 1 wherein the inert refractory materialis a calcined kaolin clay.

4. A process according to claim 1 wherein the particulate material to betreated is a kaolinitic clay mineral.

5. A process according to claim 4 wherein the fluidized bed is heated toa temperature in the range of from about 700 C. to about l,l00 C.

6. A process according to claim 1 wherein the particulate material to betreated is fed pneumatically into the fluidized bed.

7. A process according to claim 5 wherein the particulate material to betreated is fed into the fluidized bed via a conduit passinglongitudinally through the top of stopped and a fuel is injected intothe fluidized bed and burnt therein.

10. A process according to claim 1 wherein after passing through saidheated fluidized bed, the particulate material to be heat treated passesdirectly through a second heated fluidized bed arranged above the firstfluidized bed.

2. A process according to claim 1 wherein the inert refractory materialis selected from the group consisting of sand, silica and ceramicmaterials.
 3. A process according to claim 1 wherein the inertrefractory material is a calcined kaolin clay.
 4. A process according toclaim 1 wherein the particulate material to be treated is a kaoliniticclay mineral.
 5. A process according to claim 4 wherein the fluidizedbed is heated to a temperature in the range of from about 700* C. toabout 1,100* C.
 6. A process according to claim 1 wherein theparticulate material to be treated is fed pneumatically into thefluidized bed.
 7. A process according to claim 5 wherein the particulatematerial to be treated is fed into the fluidized bed via a conduitpassing longitudinally through the top of a reactor containing thefluidized bed to a position below the surface of the fluidized bed.
 8. Aprocess according to claim 1 wherein the fluidized bed is heated byburning a fuel therein.
 9. A process according to claim 1 wherein thefluidized bed is initially heated by passing hot combustion gasestherethrough until the temperature of the fluidized bed approaches thedesired working temperature and thereafter the supply of hot combustiongases is stopped and a fuel is injected into the fluidized bed and burnttherein.
 10. A process according to claim 1 wherein after passingthrough said heated fluidized bed, the particulate material to be heattreated passes directly through a second heated fluidized bed arrangedabove the first fluidized bed.